Methods of coating interiors of pipes, tubes and cylinders are known. Such methods are important where the expense of the coating material, or the physical characteristics of the coating material, prohibit the construction of the entire pipe from the coating material.
Coated pipes are typically used to convey corrosive or abrasive liquids, slurries or the like. Products within which coated tubes or cylinders are used include, shock absorbers, McPherson struts, combustion engine cylinder liners, bushings, hydraulic cylinders, oil well pipe, food process piping, nuclear power plant piping, desalinization plant piping, refinery piping, chemical manufacturing, couplings, extrusion barrels (dies), etc.
Chromium or other metals or metal alloys that resist corrosion and wear or provide a good bearing surface are good coating candidates. In strings of pipe used in deep oil wells, for example, it is desirable that the interior surface of the pipe have good resistance to corrosion and wear, so as to extend the time period before failure causing disruption of oil production and removal of the pipe string for replacement. Similarly, strings of pipe which are used to transport concrete slurry from a source of supply to the site of use, must have a wear resistance inner surface in order to withstand the abrasion caused by the aggregate (sand, gravel, and crushed stone) mixed with the cement in the slurry.
It has long been known that ordinary steels may be chrome plated, or the like, to meet surface character requirements for exposure to harsh environments. Chromium, however, is a relatively expensive material producing environmentally detrimental byproducts. Chromium is also difficult to plate onto interior surfaces of tubes.
Other coatings, such as those applied in the form of powders and later fused to a substrate, are also known. Chrome alloys, for example, may be used as a coating in many applications using methods developed for such purpose. Such methods typically include dispersing a coating material inside a spinning pipe, typically using compressed air, and heating the pipe to sufficient temperature as to fuse the coating, but not melt the pipe.
U.S. Pat. No. 4,490,411 to Feder (Feder) is an example of such a process. Under the '411 patent a powdered metallic coating material is delivered to the interior of the tubing through a spray nozzle using a compressed non-oxidizing gas. The tubing is rotated during delivery of the coating material and is heated above a fusion temperature of the coating material using an induction heating process. The fused coating then coats the interior of the pipe.
Because of the spinning, the length of tube that can be practically coated by the Feder process is limited. The process is limited because the nozzle delivering the coating material to the inside of the tube can not be allowed to touch the spinning sidewalls of the tube. Where touching occurs, either the spray distribution of coating material is disrupted or the torque occasioned by the contact causes twisting failure of the structure supporting the nozzle.
A somewhat similar process is described in U.S. Pat. No. 5,059,453 to Bernsten. (Applicant notes that he is the inventor of U.S. Pat. No. 5,059,453 and his name was misspelled in the patent. His name Bernstein is used hereafter in this discussion.) In Bernstein the coating material was delivered to the interior of the tubing by inserting metal rods into the tubing. Induction heating of sufficient intensity to fuse the rods is then applied to the tubing as the tubing is rotated at a high rate of speed.
While the coating processes described in Feder and Bernstein may be effective, the distribution of coating material is dependent upon the degree of fluidity of the coating material and rate of spinning of the tube. To achieve an even distribution of coating material, the metal rods of Bernstein must be completely fused for the coating to flow in such a manner as to cover the interior of the pipe and bridge coating gaps. The nozzle of Feder is similarly dependent upon a nozzle geometry for an even distribution of coating materials and fluid flow of melted coating materials to achieve a consistent coating.
Where a tube is not straight or is out of round, spinning cannot be relied upon for an even distribution of coating material and, in fact, causes variation in coating thickness. Portions of an interior of a tube that are close to an axis of rotation will receive very little coating material whereas portions that are relatively far from the axis of rotation will receive a heavier coating.
Accordingly it is an object of this invention to provide a means of coating tubing interiors that provides a more consistent coating thickness than the prior art.
It is a further object of the invention to provide a method of coating tubing interiors that is not dependent on the fluid flow of a coating material for coverage of the tubing interior.
It is a further object of the invention to provide a method of randomly distributing a coating over a tubing interior that is not dependent upon the placement of coating rods.
It is a further object of the invention to provide a method of randomly distributing a coating over a tubing interior that is not affected or limited by the length of the tube.